Polyphenylenes are among the simplest aromatic polymers that it is possible to prepare, and in fact, they may be thought of as the aromatic homologues of polyethylene. While such polymers are commonly characterized by superior thermal stability, due to their aromaticity, unfortunately they tend to be highly crystalline in nature, and therefore, insoluble in most organic solvents. Such insolubility substantially interferes with the ability to chemically modify the polymers, and to employ them in carrying out further, desirable reactions. Furthermore, the use of insoluble polymers is constrained, for example, in that they cannot be used to form cast films from solution, or be employed as monomeric oligomers in preparing still other valuable polymers.
In view of their advantageous characteristics, it has long been an objective of polymer chemists to develop a polyphenylene synthesis leading to soluble polyphenylene products. In the past, such attempts have included efforts to enhance the solubility of such products by the attachment of lateral substitutents, Kern et al., Makromol. Chem., 1960, 37, 198. In addition, soluble high molecular weight phenylated poly (p-phenylene)s have been synthesized by the Diels-Alder reaction of bistetracyclones with p- or m-diethynylbenzene, Stille, Macromolecules, 1971, 4, 515; Mukamal et al. J. Polym. Sci., Part A, 1967, 5, 2721; and Stille et al., Macromolecules, 1968, 1, 431.
Additional classes of soluble poly(p-phenylene)s have been obtained through the polymerization of 2,5-dibromo-1,4-di-n-alkylbenzenes, Rehahn et al., Polymer, 1989, 30, 1054, and by the polymerization of 2,5 dibromophenyl, Noll et al., Makromol. Chem., Rapid Commun., 1990, 11, 485, both the preceding using the Yamamoto reaction, i.e., Pd(0) catalyzed polymerization of 4-bromo-2,5-di-n-hexylbenzeneboronic acid, Rehahn et al., Polymer, 1989, 30, 1060, and Rehahn et al., Makromol. Chem., 1990, 191, 1991; by the anionic polymerization of 2-phenyl-1,3 cylohexadiene followed by aromatization, Noll et al., ibid; and by spontaneous polymerization of 1-bromo-4-lithiobenzene in hexamethylphosphoramide, Tour et al., J. Am. Chem. Soc., 1991, 113, 2309. The polymer resulting from the 1-bromo-4-lithobenzene is soluble because it contains both para and meta linkages, Tour et al., ibid.
Highly branched polyphenylenes can be synthesized by cocyclotrimerization of p-diethynylbenzene with phenylacetylene, Hergenrother, J. Macromol. Sci.-Rev. Macromol. Chem., 1980, C19, 1, and by Pd(0) catalyzed homocoupling of (3,5-dibromophenyl) boronic acid, such polymer also being soluble, Kim et al., J. Am. Chem. Soc., 1990, 112, 4592.
Furthermore, soluble polyarylenes containing binaphthylene structural units have been synthesized by cation-radical polymerization of bis(1-naphthyl) biphenyls, Percec et al., J. Polym. Sci., Part A; Polym. Chem. Ed., in press. The bulky and twisted binaphthylene structural units are believed most likely responsible for the solubility of the resulting polymers.
While soluble polyphenylenes have previously been prepared by a number of processes, including those described in the preceding, such processes have typically been relatively complex, in many cases requiring excessive processing steps. Furthermore, the starting materials have in numerous instances not been commercially available. These and other factors have, consequently, limited the usefulness of the procedures previously known.